The invention relates to a method and apparatus for bonding substantially rotationally symmetric discs, in particular discs made from polycarbonate, which have a center opening. The present invention is particularly useful for bonding two DVD halves; here, two halves of an information carrier, each half having a thickness of 0.6 mm, are to be bonded. The halves are held apart so that a gap is created between the discs; after the adhesive is injected into the gap, the two disc halves rotate such that the adhesive is spread across the two disc halves until the disc halves are completely covered with the adhesive. Certain problems occur in the bonding of various DVD variants with a diameter of 120 mm (a diameter of 80 mm is also feasible), such as DVD 5, 10, 9 and DVD 18 in DVD audio, DVD video, DVD-ROM, DVD-R and DVD-RAM configurations. These problems can be due to unevenness (radial tilt and tangential tilt) in the radial and tangential directions as well as to contamination during the bonding process, for example, if the adhesive layer is non-uniform or contains bubbles. Bubbles in the adhesive layer are extremely critical and cause the DVD to fail. This problem is exacerbated in the DVD 9, which stores information in two planes, wherein the second information plane is read out through the adhesive layer.
Several conventional methods are used to manufacture DVD""s. In the so-called xe2x80x9chot meltxe2x80x9d bonding process, the adhesive is hot and highly viscous and is applied to each DVD half with a roller. The two halves are subsequently superimposed and pressed together. Bubbles can form very easily with this process, causing flaws in the adhesive layer and making this method useless for the manufacture of a DVD 9. In the xe2x80x9cUV screen print bondingxe2x80x9d process, adhesive is applied to both halves (similar to hot melt bonding), and the halves are subsequently superimposed and pressed together. Unlike the xe2x80x9chot melt bondingxe2x80x9d process, a screen print process is used herein to apply the adhesive. The xe2x80x9chot melt bondingxe2x80x9d process, however, poses the same problems as the xe2x80x9chot melt bondingxe2x80x9d process, i.e. the adhesive layer has flaws and bubbles are formed. Moreover, the unevenness of the disc (radial tilt, tangential tilt) can create problems. This process is described in a publication from the company EDME.
Another process is xe2x80x9cUV capillary bondingxe2x80x9d as described in a product brochure from the company Toolex Alpha. The two DVD halves are in this process held apart by vacuum discs and are simultaneously bent (FIGS. 4-5 in the brochure). The adhesive is injected through a dispenser needle which is inserted into the gap between the two DVD halves. The dispenser needle extends radially to about half the radius, at which location adhesive is furnished during a single revolution of the disc. The dispenser needle is then withdrawn from the gap and the vacuum is released. The xe2x80x9cbentxe2x80x9d DVD halves now return to their original parallel alignment and the adhesive is spread both inwardly and outwardly by capillary forces. This method has the disadvantage that cracks can develop in the sputtered metallic layers (which are used to reflect the laser light). The surfaces of the DVD halves are also susceptive to damage when the dispenser needle is inserted into the gap. Lastly, due the rotation of the DVD when the adhesive is applied, the centrifugal forces from the applied adhesive can press against the dispenser needle and cause xe2x80x9cjammingxe2x80x9d of the adhesive in front of the dispenser needle.
Most closely related to the present invention is the xe2x80x9cUV spin coat bondingxe2x80x9d process which is described in a product specification from the company Panasonic. In the xe2x80x9cUV spin coat bondingxe2x80x9d process, the two DVD halves are held parallel to each other with a gap therebetween. A strand of adhesive is applied through a dispenser needle which is inserted into the gap close to the center of the DVD. The strand of adhesive is applied close to the center during a single revolution of the disc. The dispenser needle is then withdrawn from the gap and the adhesive is spun off through rotation, thereby spreading over the two halves. With this process, the strand of adhesive supposedly adheres to both halves so that a relatively bubble-free adhesive layer is created when the adhesive is spun off. The process, however, has the drawback that the dispenser needle must be inserted close to the center. This can also result in damage to the DVD halves. The damage can be prevented by accurately supporting the two DVD halves and by precisely guiding the dispenser needle, which requires complex equipment. Moreover, when the strand of adhesive is applied and the adhesive emerges from the dispenser needles, the centrifugal force pushes the adhesive against the dispenser needle and disturbs the adhesive flow. The dispenser needle also has to be removed from the gap, before the adhesive is spread by rotating the disc, which requires an additional operational step.
It is thus an object of the present invention to provide an improved method for bonding two DVD halves, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved method for reliably bonding, without forming bubbles, two DVD halves or other rotationally symmetric discs, in particular discs made from polycarbonate, which have a center opening.
The objects, and others which will become apparent hereinafter, are attained in accordance with one aspect of the present invention for bonding substantially rotationally symmetric plates, in particular plates based on polycarbonate, having a center opening, by introducing adhesive between the spaced-apart plates through the center opening and spreading the adhesive on the plates through rotation of the plates.
In accordance with another aspect of the present invention for bonding two DVD halves spaced from one another at formation of a gap between the halves, he method includes the steps of introducing adhesive in the gap through a center opening of the DVD; and spreading the adhesive across the two halves.
In accordance with the present invention, the adhesive is introduced through the center opening, and thus can advantageously flow unimpeded from the supply element, for example a metering needle, to the outside. The adhesive then does no longer accumulate in front of the metering needle, so that subsequent process steps can be initiated sooner. The adhesive bonding process can be carried out without interruption from the time the adhesive is injected until the spinning operation is completed.
According to another feature of the present invention, the metering needle is rigid and the end of the metering needle is bent about an angle of 90xc2x0, wherein the length of the bent section is smaller than the diameter of the opening. The metering needle may suitably enter the gap between the two DVD halves only along a short distance, so that the surfaces are almost never damaged. Through adjusting an appropriate wedge angle, the adhesive disposed in the wedge is spread uniformly and symmetrically, provided that the rotational speed is increased slightly (depending on the dynamic pressure in the gap) after application of the adhesive. Before the adhesive is actually spun off, each point of the half has the same initial condition so that the flow velocity of the leading edge is uniform in all directions. Consequently, the inclusion of bubbles by leading adhesive flow is eliminated. The exit opening of the dispenser needle can be designed so that the DVD halves are wetted simultaneously when the adhesive is injected.
It is another object of the invention to provide an apparatus for carrying out the method.
This object, and others which will become apparent hereinafter are attained in accordance with the present invention by providing a retention unit for holding and moving the plates relative to one another, a centering unit for centering the plates, and at least one fluid supply line which is guided from the outside through one of the central openings and terminates in the space between the spaced-apart plates.